1. Field of the Invention
The invention relates to a waveform response improvement circuit which is used in an image signal processing of a video tape recorder or the like.
2. Description of the Related Art
FIG. 1 is a block diagram showing an example of a conventional waveform response improvement circuit, which is disclosed in, for example, Japanese Patent Application Laid-Open No. 63-292776(1988). An input signal inputted from an input terminal 33n is delayed in a first, delay circuit 33 by a fixed time D, and then further delayed in a second delay circuit 34 by the same fixed time D. In a subtracting circuit 35, the input signal is subtracted from an output signal of the first delay circuit 33, and an absolute value of the resulting signal is then obtained by an absolute value circuit 37. In a subtracting circuit 36, similarly, an output signal of the second delay circuit 34 is subtracted from the output signal of the first delay circuit 33, and an absolute value of the resulting signal is then obtained by an absolute value circuit 38.
An output signal 37a of the absolute value circuit. 37 and an output signal 38a of the absolute value circuit 38 are compared with each other in a comparator 39. On the basis of a comparison result, a changeover switch 40 is controlled so that either of the input signal (terminal a), the output signal of the first delay circuit 33 (terminal b) and the output signal of the second delay circuit 34 (terminal c) is selected, and the selected signal is outputted from an output terminal 41.
FIG. 2 is a circuit diagram showing a configuration example of the absolute value circuits 37 and 38 shown in FIG. 1. Collectors of transistors 55 and 57 are respectively connected through resistors R1 to power sources 54 and 56, and emitters of the transistors 55 and 57 are connected through resistors R2 to a constant current source 62, thereby constituting a differential amplifier circuit. A constant voltage from a constant voltage source 61 is applied to the base of the transistor 57, and an input voltage A to the base of the transistor 55.
Collectors of transistors 59 and 60 are connected to a power source 58, and emitters of the transistors 59 and 60 are connected to a constant current source 63, thereby constituting an emitter follower circuits. An output of abovementioned differential amplifier circuit is inputted to the bases of the transistors 59 and 60. Specifically, a noninverted signal of the input voltage A is input ted to the base of the transistor 59, and an inverted signal of the input voltage A to the base of the transistor 60.
When the input voltage A is positive, therefore, a positive noninverted signal of the input voltage. A is inputted to the base of the transistor 59, and a negative inverted signal of the input voltage A to the base of the transistor 60, resulting in that only the transistor 59 to which a higher voltage is inputted is turned on, and the positive noninverted signal of the input voltage A is outputted from the emitter. In contrast, when the input voltage A is negative, a negative noninverted signal of the input voltage A is inputted to the base of the transistor 59, and a positive inverted signal of the input voltage A to the base of the transistor 60, resulting in that only the transistor 60 to which a higher voltage is inputted is turned on and the positive inverted signal of the input voltage A is outputted from the emitter. Irrespective of the polarity of the input voltage A, consequently, the absolute value of the input voltage A is outputted.
An operation of the conventional waveform response improvement circuit thus configured will be described.
When the output signal of the first delay circuit 33 is indicated by f(t), the input signal at the input terminal 33n is expressed by f(t-D), and the output signal of the second delay circuit 34 by f(t+D). FIG. 3(a), FIG. 3(b) and FIG. 3(c) or FIG. 4(a), FIG. 4(b) and FIG. 4(c) show a timing chart illustrating relative positional relationships of these signal examples. The signals f(t-D), f(t) and f(t+D) are shifted by the fixed time D in this example.
The subtracting circuit 35 every time calculates [amplitude of signal f(t)]-[amplitude of signal f(t-D)], and the absolute value circuit 37 outputs the signal 37a (FIG. 3(d), FIG. 4(d) indicative of the absolute value of a calculation result. Similarly, the subtracting circuit 36 every time calculates [amplitude of signal f(t)]-[amplitude of signal f(t+D)], and the absolute value circuit 38 outputs the signal 38a (FIG. 3(e), FIG.4(e)) indicative of the absolute value of a calculation result.
The comparator 39 compares the absolute value signal 37a with the absolute value signal 38a. As shown in a switching operation of FIG. 3(f) or FIG.4(f), when the signal 37a is smaller than the signal 38a (37a&lt;38a), the changeover switch 40 is switched to the terminal a so that the signal f(t-D)) is outputted, when the signal 37a is equal to the signal 38a (37a=38a), the changeover switch 40 is switched to the terminal b so that the signal f(t) is outputted, and when the signal 37a is larger than the signal 38a (37a&gt;38a), the changeover switch 40 is switched to the terminal c so that the signal f(t+D) is outputted.
As a result, a signal g(t) shown in FIG. 3(g) or FIG. 4(g) is outputted from the output terminal 41. The signal g(t) has a waveform obtained by correcting the color signal f(t) having a slow (gentle) rising to a waveform having a rapid (steep) rising. At the rising of the color signal, therefore, the rising time is shortened, and, at the falling of the color signal, the falling time is shortened.
In the conventional waveform response improvement circuit, there arises no problem when the amplitude of the input signal is sufficiently large. When the amplitude of the an input signal is small and lower than the threshold voltages of the comparator and the changeover switch, however, the circuit may erroneously operate. In the case where an area (area=amplitude=time) of the input signal is small, for example, the input signal f(t) shown in FIG. 4(b) may be changed to a signal g(t) shown in FIG. 3(g), thereby causing decoloration. In the current status of the art, a waveform response improvement circuit cannot always operate normally and fails to sufficiently exhibit its function.